Polymerizable mesogenic or liquid crystal (LC) compounds, which are also known as reactive mesogens (RM), have been described in prior art for various purposes. For example, they can be used for the preparation of linear or crosslinked LC side chain polymers. Furthermore, they can be aligned in their LC phase and subsequently polymerized in situ to give linear or crosslinked LC polymer films with uniform orientation of high quality. These films can be used as optical elements like polarisers or compensators in flat panel displays, as described for example in EP 0 397 263, WO 98/00475, WO 98/04651 or WO 98/12584.
The polymerizable mesogenic compounds described in prior art, however, often exhibit LC phases only in a small temperature range or do not show mesophase behavior at all. Another drawback for specific applications is that prior art compounds often exhibit high values of the birefringence.
In particular where polymerizable mesogenic compounds are used in optical phase retardation, compensation or alignment layers or films for LC displays (LCD), it is desirable to have available materials of which the optical properties, such as the birefringence, optical retardation and optical dispersion (wavelength dependence of the birefringence) can be adapted to those of the LC mixture in the display cell, so that optimum compensation can be achieved.
Further to the use as optical films in displays, polymerizable mesogenic compounds have been suggested for use in the active, switchable layer of an LCD.
For example, displays are known which are switched between transparent and scattering states and comprise a low molar mass liquid crystal (LC) medium and a phase-separated polymerised LC material, like for example PDLC (polymer dispersed liquid crystal) displays as described in WO 93/22397, or polymer gel or polymer network displays of the scattering type, as described in U.S. Pat. No. 5,538,768, U.S. Pat. No. 5,543,075 or EP 0 451 905.
Furthermore, displays are known wherein a low molar mass LC medium is switched between two non-scattering states, like conventional displays of e.g. the TN or STN (twisted nematic, supertwisted nematic), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane-switching) mode, and which further comprise a polymerised LC material in order to create a multidomain structure for improving the contrast at wide viewing angles or to stabilize the different switching states for reducing the driving voltage and switching times. Such displays are described for example in U.S. Pat. No. 5,189,540, U.S. Pat. No. 6,177,972, EP 0 903 392, and Hasebe et al., Jpn. J. Appl. Phys. 1994, 33, 6245. For use in the switchable layer of LCDs the polymerizable compounds are typically mixed with a low molar mass LC medium. Therefore it is required that the p F, Cl, Br, I, CN, NO2 or alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl, alkylcarbonyloxy or alkoxycarbonyloxy with 1 to 20 C atoms, wherein one or more H atoms may be substituted by F or Cl
Polymerizable compounds do not negatively affect the properties, like the LC phase range, birefringence and dielectric anisotropy of the LC medium. For use in switchable displays that are not of the scattering type, and where the polymerised material is not macroscopically phase separated from the low molar mass LC medium, the polymerizable compounds should further show good miscibility with the LC medium.
The polymerizable mesogenic compounds described in prior art, however, do often exhibit high melting points, show poor solubility in low molar mass LC media or tend to negatively influence the LC and electrooptical properties of LC media.
Thus, there is a demand for polymerizable mesogenic compounds with low melting points which are useful for the preparation of oriented LC polymer films as optical retarders, and for the use in the active layer of switchable LC devices, for example in mixture with low molar mass LC media where they should have optical and electrooptical properties adapted to those of the LC media and good solubility in the LC media.
Furthermore, regarding the broad range of applications for polymerizable compounds it is desirable for the expert to have available further compounds of this type that are easy to synthesize and fulfill the various requirements as described above.
It was an aim of the invention to provide new polymerizable mesogenic compounds with advantageous properties as mentioned above, thus extending the pool of polymerizable mesogenic compounds available to the expert. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing polymerizable mesogenic compounds as claimed in the present invention, comprising a terminal fluorinated phenyl group.
JP-A-07-069983 discloses polymerizable mesogenic compounds, including 4-(3,4-difluorophenyl)cyclohexyl acrylate and 4-(trans-4-3,4,5-trifluorophenyl)cyclohexyl) cyclohexyl acrylate, but does not specifically disclose compounds according to the present invention.